Fluvial And Fossil Sediments

Waterborne or fluvial sediments include fine materials resembling aeolian deposits, but also larger particles such as gravel, pebbles, cobbles, and organic debris too dense or massive to be entrained by winds. Materials drop out of the water column in order of density; the denser the object, the more quickly it is deposited. As one result, the densest fluvial sediments accumulate closest to the point where the river or stream that carries them enters a lake, producing horizontal sorting patterns called facies.

As another result, larger and smaller particles that arrive together will not settle out at the same rates. In regions where runoff is seasonal, a pattern of alternating bands of coarse, dense materials and finer, lighter ones will develop. Once their seasonality is understood, such varved sediments provide a year-by-year account of relative precipitation on a particular watershed.

Some sediments accumulate in lakes because small, short-lived organisms reproduce and die off in large numbers. For paleolimnologists, some of the most useful organic sediments include the persistent shells and scales of diatoms, chrysophytes (golden-brown algae), mollusks, arthropods, and foraminifera. Diatoms are algae that develop hard, silicon structures called frus-tules, which can be used to identify different species. The presence of different diatom species in different sediments has been used as a proxy for variations in pH (acidity), temperature, salinity, nutrient concentration, and even the annual period of ice cover. Many invertebrate animals and smaller foraminifera secrete calcium carbonate- (CaCO3) based shells.

In addition to species-based ecological proxies similar to those provided by diatoms, the carbon (C) content of these shells allows sediments up to about 60,000 years old to be dated using the radiocarbon (14C) technique familiar to archaeologists. However, diatom frustules and foraminiferan shells are small enough to become windborne, and their presence in lake sediments must be interpreted with caution.

Leaves and other common plant remains composed of cellulose can also be dated with radioactive decay analysis of the unstable isotopes 513C and 518O (oxygen). Plant remains may also be blown or washed into lakes, so materials from identifiable species provide more precise proxies for climate effects. But, even the presence of pigments such as chlorophyll and carot-enoids in sediments can provide a general proxy, indicating the growth of plants. Finally, any identifiable fossil remains of insects, fish, and other animals can provide further clues to the sorts of environments that existed at any particular time.

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